This invention relates broadly to computer memory systems and more particularly to such systems organized from circular shift registers which require periodic refreshing.
The advantages obtainable by organizing memory systems comprising loops of charge storage elements, for example, charge coupled devices or MOS devices, are well-known. Basically, high packing densities and rapid access are obtainable. While these advantages appear, several complications arise when one attempts to organize a memory system from charge storage shift register loops.
These complications arise from two competing concerns. First, it is necessary to intermittently refresh the contents of the charge storage elements. Such refresh is accomplished by circulating the shift register contents. Each such shift results in power dissipation in the memory system. Therefore, it is desirable to keep such circulation to a minimum. On the other hand, it is desirable to have immediate access to the data stored in the circular shift registers whenever the associated computer system demands access. A conflict may arise because the computer system may demand access to data stored in shift registers which are currently being circulated for refresh purposes. Thus, there exists a critical problem in synchronizing and coordinating the refresh and access operations in order to provide maximum flexibility, minimum power dissipation and errorfree operation.